An analog-to-digital converter is a ubiquitous component found in many different types of systems, such as but certainly not limited to computer, data, control, sensor, communication, and telecommunication systems. The analog-to-digital converters receive analog signals and provide a digital signal to another component, such as a processor. As the systems within which the analog-to-digital converters are used develop and become more sophisticated, the performance of the analog-to-digital converter is becoming more important.
For instance, as the development of a software defined radio continues, the need for high-speed analog to digital conversion to directly digitize RF to microwave signals rather than to down-converting the signal to IF becomes necessary to improve performance, to simplify design, to reduce noise, to reduce interference, and to reduce cost. The current technology of analog to digital conversion is primarily done at the electronic signal. The high speed sampling of the electronic signal has been limited to the stability of the clock jitter, thermal noise of the electronics, electromagnetic interference of other electronic devices and circuits, cross-talk, and coupling noises of interconnect lines.
The use of an optical signal has benefited many high speed communication applications due to the inherent inertness of the optical signals to the EMI noise and minimal cross-talk and coupling to close by devices. Optical signals can also travel relatively long distances without compromising severe signal distortion and attenuation at high modulation speed.
To obtain even better performance, some work has been done in performing the analog-to-digital conversion in the optical domain. a photonic analog-to-digital converter can take advantage of the high speed analog signal of the optical domain and convert it to a high speed digital signal in the electronic domain for further signal processing. The photonic analog-to-digital converter can therefore achieve high speed analog to digital conversion beyond today's technology. The photonic analog-to-digital converter can provide a system with the low noise, low distortion, and high-speed characteristic of photonics while leveraging the more established high-speed digital electronics for low cost signal processing. The digital signal is also less sensitive to noise and can be processed using today's semiconductor technology at relatively high speed.
FIG. 1 illustrates an example of a photonic analog-to-digital converter 10 using a conventional optical splitter 12, saturable absorbers 14, optical delay lines 16, photo-diode detectors 17 and electronic comparators 19. With this analog-to-digital converter, incoming photonic signals are split into a plurality of photonic components and directed toward the saturable absorbers 14. The saturable absorbers 14 quantize the photonic signal by preventing any signal from reaching the photo-diode detectors 17 unless the photonic components exceed a certain threshold level. The comparators 19 then digitize the output by comparing the output of the photo-diode detectors 17 to a reference level for the digital signal. A digital encoder 18 combines the individual bits from the comparators to form a digital signal.
A problem with this approach is that the use of conventional optical waveguides 16 and/or fiber splitter 12 makes it difficult to miniaturize the analog-to-digital converter 10. This is due to the minimum-bending radius allowed in order to minimize the scattering losses at the bends of the waveguides 16 and splitter 12. Secondly, the use of saturable absorbers 14 to quantize the optical signal creates a highly inefficient conversion. Most of the original optical signal will be wasted through absorption. As number of optical split channels required for the number of digital bit resolution is channels=2n bits, the device 10 quickly becomes impractical to implement.